This invention relates to cathode ray tube image displays, hereafter simply called "CRT's," and is addressed to means for suppressing stray magnetic fields emitted by such systems. More particularly, the objective is to suppress such emissions to a level below that established by a recognized standard. The invention is applicable to both monochrome and color CRT image displays in which the viewer may be in close proximity to the faceplate, such as the viewers of visual display terminals and television sets.
The present invention had its origin in the concern over the possible detrimental effects of stray magnetic fields on the physiology of the viewers. Testing for such fields in visual display terminals is described in a publication of the National Board for Measurement and Testing (MPR) of Sweden entitled "Test Methods for Visual Display Units: Visual Ergonomics and Emission Characteristics." MPR 1990:8 1990-1991, Boras, Sweden. This standard is known as "MPR-2." The subject of stray magnetic fields is also covered by the IEEE Transactions on Electromagnetic Compatibility, a periodic publication of the IEEE Electromagnetic Compatibility Society.
As is known, the primary source of stray magnetic fields in CRT's is the yoke. The yoke is an electromagnetic device that causes an electron beam to scan a raster on the CRT viewing screen in both the horizontal and vertical directions. Monochrome CRT's utilize a single electron beam. CRT's displaying color images typically utilize three beams, one for energizing each of the red, green and blue light-emitting phosphors on the viewing screen. In this disclosure, reference is made to only a single beam CRT, with the understanding that its content applies as well to multiple-beam CRT's.
Essentially, a yoke consists of two pairs of coils, one of which deflects the electron beam in the horizontal direction, and the other in the vertical direction. The two pairs of coils appear as dual radiating magnetic dipoles. The present invention is directed to the suppression of stray magnetic fields produced by the horizontal deflection yoke coils.
The coils are energized by a horizontal oscillator and a vertical oscillator. The horizontal oscillator provides a train of pulses having a frequency of 15,750 Hz in monochrome television sets, a frequency of 15,734.26 Hz in color television sets, and frequencies of up to 150 kHz in some visual display terminals. The pulses are routed to the horizontal winding of the yoke.
FIG. 1A depicts the general configuration of the sawtooth waveform pulse emitted by a horizontal oscillator. The activation of prior art stray magnetic field suppression devices has been accomplished by the inverted sawtooth waveform pulse indicated in FIG. 1B, which is essentially a mirror-image of the horizontal oscillator waveform of FIG. 1A. The inverted pulse depicted is used to energize prior art stray field suppression devices such as the device disclosed in U.S. Pat. No. 4,709,220 to Sakane et al.
Known solutions to the problem of suppressing stray magnetic fields typically involve some shielding combined with field cancellation techniques. The techniques have taken the form of additional field generating coils in series connection with the horizontal deflection windings of the yoke. The disadvantage of this approach is that higher pulse voltages are required to drive the windings. As a result, a major redesign of the circuits of the horizontal oscillator and the power supplies has been required.
The stray magnetic field that emanates from the beam-deflecting yoke of CRT is depicted diagrammatically in FIG. 2. A CRT 10 is enclosed in a cabinet 12. A beam-deflecting yoke 14 is indicated as encircling the neck 16 of CRT 10. By the variance of its magnetic fields, yoke 14 provides for the horizontal and vertical deflection of an electron beam emitted by an electron gun 18 that is enclosed in neck 16 of CRT 10.
The stray magnetic field that emanates from the horizontal deflection coil of the yoke 14 is indicated as being composed of two loops, a first loop 22 and a second loop 24, both of which extend beyond the perimeter of the cabinet 12. The clockwise direction of the stray magnetic field indicated by first loop 22 that extends into the frontal area 26 of cabinet 12 is indicated by arrows 28 and 30. The counter-clockwise direction of the stray magnetic field indicated by second loop 24 that extends into the rearward area 32 of cabinet 12 is indicated by arrows 34 and 36.
The suppression of the stray magnetic field by a stray field suppression system is indicated diagrammatically in the form of two loops 38 and 40 running in paths opposite to the respective paths of the stray fields indicated by loops 22 and 24. The stray field represented by first loop 22, shown as rotating in a clockwise direction indicated by arrows 28 and 30, is opposed by the third loop 38, in which arrows 41 and 42 indicate that the stray field suppressing third loop 38 lies in a counterclockwise direction. Similarly, the stray field indicated. by second loop 24 is opposed by the fourth loop 40, and arrows 44 and 46 indicate that the direction of the fourth loop 40 lies in a clockwise direction.
The measurement of the strength of the magnetic field emitted by the yoke 14 of the CRT 10 is indicated in FIGS. 3 and 4. The value measured is the rms (root mean square) of the strength of the field according to the formula for the rms value of periodic waveforms: ##EQU1##
FIG. 3 is a three-dimensional view of the three planes of the system along which the field is measured: a top plane 48, a middle plane 50 (also shown by FIGS. 2 and 4) and a bottom plane 52. The distance 54 between the planes is preferably 0.3 meter.
With reference to FIG. 4, measurements are made from the center 54 of the cabinet 12. Center 54 is in coincidence with the horizontal center line 56 of CRT 10. The distance R, or radius, in meters between the center 54 of the cabinet 12 and the perimeter of the three planes 48, 50 and 52 is determined by the formula R=L/2+0.5 m, where L is the front-to-back dimension of the cabinet 12.
FIG. 4 also depicts the points of measurement 58 of the strength of a stray magnetic field on each of the three planes 48, 50 and 52. Measurements on each plane are taken every 22.5 degrees. As sixteen measurements are taken in each plane, the total number of measuring points is forty-eight.
The range of stray field intensity among CRT image displays of various sizes and types was found to be 70 to 150 nT (nanoTesla). Measurements at the measurement points on planes 48, 50 and 52 disclosed a range of 40 to 150 nT. The Swedish MPR-2 standard specifies a maximum of 25 nT. The strength of a stray magnetic field is determined by means of a meter capable of measuring the rms value of low frequency magnetic fields; that is, fields in the frequency range of 2,000 Hz to 400 kHz. The meter must have a dynamic range of 0.01 uT to 10,000 uT. The measurement cycle includes measurement of the strength of stray magnetic field, its frequency and its polarization. A suitable instrument is Magnetic Field Meter 1000 manufactured by Combinova AB, Bromma, Sweden. The United States representative of this company is Ergonomics, Inc., Southhampton, Pa.
A feasible system for the resolution of the problem of stray magnetic fields must provide for stray field suppression well below the established standard, and do so with minimum interference with existing circuits, and with minimum power consumption. Also, the system must be simple and inexpensive to manufacture and install. The present invention meets these and all other requirements.